Glycolysis and Gluconeogenesis II

How is glycolysis controlled in skeletal muscle?

-ATP production for contraction -Energy status is important

How is glycolysis controlled in the liver?

-Regulation of blood glucose levels -Glucose levels are important

Enzymes that catalyze irreversible steps are controlled mainly by three different mechanisms: 1. 2. 3.

1. Reversible allosteric control - milliseconds 2. Regulation by reversible phosphorylation - seconds (by hormones) 3. Transcriptional control - hours

______________ is a non-metabolizable glucose analog that is used to visualize tumors on PET scans. It is taken up by glucose transporters but not metabolized.

2-18FD-deoxyglucose.

Glucose transporters GLUT1- GLUT5 consist of about _______ amino acids and have ________ transmembrane domains.

500 12.

Glycolysis in Skeletal Muscle is Regulated to Meet the Need for ATP: Allosteric inhibition by ATP is reversed by ___________.

AMP.

Contracting muscle uses ATP for contraction and ATP levels decrease, while __________ levels increase, which activates PFK. Activation of PFK during muscle contraction increases the levels of _______________, which activates ____________.

AMP. fructose 1,6-bisphosphate. pyruvate kinase.

Pyruvate kinase is allosterically inhibited by high levels of _____________ in skeletal muscle.

ATP (when energy charge is high) and alanine (when bulding blocks are abundant).

Phosphofructokinase in liver is allosterically inhibited by high levels of ______________.

ATP and citrate.

Muscle contracts and requires energy in form of __________. Liver regulates _____________ levels during fasting.

ATP. blood glucose.

Low pH inhibits PFK activity by augmenting the inhibitory effect of _______. Contracting muscle produces ________ which lowers the pH. To avoid damage, PFK is inhibited when pH drops due to lactate accumulation.

ATP. lactate.

Glycerol enters gluconeogenesis as ____________.

DHAP.

The production of phosphoenolpyruvate from pyruvate occurs in two steps in gluconeogenesis.

First, pyruvate is carboxylated to oxaloacetate by pyruvate carboxylase. Oxaloacetate is then decarboxylated and phosphorylated by PEP-carboxykinase to PEP.

_____________ activates the pyruvate kinase.

Fructose 1,6-bisphosphate.

_______________ is a potent allosteric activator of phosphofructokinase in the liver (not in muscle).

Fructose 2,6-bisphosphate (F-2,6-BP).

HIF-1 stimulates the expression of ______________ and glycolytic genes, giving cancer cells a growth advantage.

GLUT1 and GLUT3.

Which glucose transporter has a very high affinity for glucose and functions when blood glucose levels are normal to continuously take up glucose?

GLUT1 and GLUT3.

_______________ are present in most mammalian cells and are responsible for basal glucose uptake (KM= 1 mM). At normal blood glucose levels they transport glucose into the cells.

GLUT1 and GLUT3.

Which glucose transporter functions after a meal because it requires high levels of glucose since it has a lower affinity for glucose?

GLUT2.

____________ is present in liver and pancreatic beta-cells and has a very high KM for glucose (15-20 mM).

GLUT2.

Insulin promotes glucose uptake into muscle and fat. ___________ is the insulin-responsive glucose transporter.

GLUT4.

___________ has a KM of 5 mM and mediates the entry of glucose into muscle and fat cells.

GLUT4.

____________ is present in the small intestine and functions as a fructose transporter.

GLUT5.

In the liver, _______________ phosphorylates glucose only when it is abundant and has a 50-fold lower affinity for glucose than hexokinase (Km=5mM vs. 0.1mM).

Glucokinase.

_______________ is especially important during longer periods of fasting (6-8 hours) or starvation.

Gluconeogenesis.

_______________ provides glucose after prolonged fasting when hepatic glycogen stores are low.

Gluconeogenesis.

Why is hexokinase not the primary control site in glycolysis?

Glucose 6-phosphate is also used in glycogen synthesis and in the pentose phosphate pathway. **this step is not unique to glycolysis.

_____________ is obtained from hydrolysis of triacylglycerols in adipose tissue, which become a major energy source during fasting or starvation.

Glycerol.

_____________ is the primary source of blood glucose between meals.

Hepatic glycogen.

Which of the following conditions would directly lead to activation of phosphofructokinase and thereby increase the rate of glycolysis in muscle?

High levels of AMP.

GLUT4 is present in muscle and fat cells. It is sequestered within the cell in vesicles. ___________, which signals the fed state and is secreted from the pancreas after each meal in response to high blood glucose levels, leads to translocation of GLUT4 to the membrane and thereby increases glucose uptake into muscle and fat.

Insulin.

When blood glucose levels are low, the glucagon-triggered cAMP cascade leads to phosphorylation and inhibition of _______-type pyruvate kinase.

L-type **This regulation prevents liver from using glucose when brain and muscle urgently need it.

The oxaloacetate that is generated in the mitochondria cannot be directly transported out of the mitochondria and is reduced to malate inside the mitochondria for transport to the cytosol by ______________. When malate is in the cytosol, it is reoxidized to oxaloacetate by ______________. Oxaloacetate is then decarboxylated and phosphorylated by phosphoenolpyruvate (PEP) carboxykinase to phosphoenolpyruvate.

NADH-linked malate dehydrogenase. NAD+ linked malate dehydrogenase.

Is gluconeogenesis the reverse of glycolysis?

No, has some shared pathways but also separate rxns.

In gluconeogenesis, the conversion of pyruvate into phosphoenolpyruvate (PEP) begins with the formation of ______________.

OAA.

At rest, muscle ATP levels are high, which inhibits _______________. Inhibition of PFK leads to accumulation of ____________, leading to accumulation of ____________, which inhibits hexokinase.

PFK and pyruvate kinase. fructose-6-P. glucose 6-P

AMP competes with ATP and reverses inhibition of _____________.

PFK by ATP.

____________ is the major enzyme that regulates the rate of glycolysis.

PFK.

_____________ is the major control element of glycolysis. It is allosterically inhibited by high levels of ___________ (reduced the affinity for substrate).

Phosphofructokinase (PFK). ATP.

What advantages do tumors gain from aerobic glycolysis?

Production and secretion of lactate makes the tumor environment acidic and enables tumor invasion and inhibits the immune system from attacking the tumor.

In gluconeogenesis, the pyruvate kinase reaction of glycolysis is bypassed by _____________.

Pyruvate Carboxylase Phosphoenolpyruvate Carboxykinase

______________ has a L-type that predominates in liver, and a M type in muscle and brain.

Pyruvate kinase.

___________ muscle uses FA's and glucose from glycogen degradation in the liver.

Resting.

Forcing tumor cells to carry out oxidative phosphorylation kills the cells.

True, cancer cells generate ATP only by carrying out glycolysis and lactic acid fermentation even if oxygen is available.

At normal blood glucose levels, most of glucose is taken by the brain and the amount of glucose entering the liver is minimal.

True, liver mostly uses FA's and saves glucose for use by the brain. Glucose mainly only goes to the liver after a meal when bg levels are high.

Rapidly growing tumors obtain ATP by converting glucose to lactate even in the presence of oxygen.

True, this is called aerobic glycolysis (or the Warburg effect).

Low pH is not a regulator of phosphofructokinase in liver.

True.

The three irreversible reactions of glycolysis are bypassed in gluconeogenesis by new steps.

True.

Whole body needs about 160g of glucose daily and 120g of it is used by the brain.

True.

Glucose enters liver and pancreatic beta cells only if it is ___________, such as after a meal.

abundant.

In liver glycolysis is accelerated when glucose is ____________.

abundant.

High levels of ____________ are indicative of high levels of building blocks. Thus, there is no need to metabolize glucose to make more building blocks.

alanine.

When glucose supply is limited, glucose goes first to the ______________.

brain and muscle.

Glycerol (from hydrolysis of triacylglycerols) is converted into _______________ and enters gluconeogenesis or glycolysis as ________________.

dihydroxyacetone phosphate. glyceraldehyde 3-phosphate.

Glycerol enters gluconeogenesis as glyceraldehyde 3-phosphate after isomerization of _____________ to glyceraldehyde 3-P by _______________.

dihydroxyacetone phosphate. triose phosphate isomerase.

Citrate is an important precursor in liver (citrate is used in ______________) but not in muscle.

fatty acid synthesis.

High levels of ______________ suggest that stage 1 of glycolysis is active.

fructose 1,6-BP.

Phosphoenolpyruvate is converted to _________________ by the same enzymes as used in glycolysis, because they catalyze reversible reactions.

fructose 1,6-bisphosphate.

High concentrations of ATP convert the hyperbolic binding curve of ____________ into a sigmoidal one.

fructose 6-phosphate.

High blood-glucose levels increase _____________ levels accelerating the synthesis of ____________. Thus, when glucose is abundant, the rate of glycolysis is increased via increasing the levels of F-2,6-BP.

fructose 6-phosphate. F-2,6-BP.

When phosphofructokinase is inactive, ___________ levels rise and it is converted to _____________.

fructose 6-phosphate. glucose 6-phosphate.

ATP normally functions as a substrate for PFK, however at high concentration, ATP binds to a regulatory site within PFK and inhibits PFK by reducing its affinity for its substrate ______________.

fructose-6-P.

Liver has a special hexokinase called ______________, which is not inhibited by glucose 6-phosphate.

glucokinase.

During fasting or starvation, _______________ supplies almost all of the body's glucose and maintains normal blood glucose levels.

gluconeogenesis.

___________ is the synthesis of glucose from non carbohydrate precursors.

gluconeogenesis.

Hexokinase is allosterically inhibited by its product ____________.

glucose 6-phosphate.

Glucose 6-phosphate (generated by hexokinase) is also used in ____________ synthesis and in the _____________ pathway.

glycogen. pentose phosphate.

In glycolysis there are three irreversible steps. What enzymes catalyze these steps?

hexokinase phosphofructokinase-1 pyruvate kinase

Inhibition of phosphofructokinase (by high ATP) leads to inhibition of _____________.

hexokinase.

Pyruvate kinase activity in skeletal muscle is inhibited by ______________ and activated by _____________.

high levels of ATP and alanine. fructose 1,6-BP.

During hypoxia, _____________ increases the expression of glycolytic enzymes and glucose transporters GLUT1 & GLUT3.

hypoxia-inducible transcription factor (HIF1).

Tumors have (increased/decreased) glucose uptake and glycolysis. Tumor cells originally lack oxygen, a condition called hypoxia, which leads to activation of ___________.

increased. HIF-1.

Phosphofructokinase is also controlled by pH. When pH drops the enzyme is _____________.

inhibited. **only in muscle, not liver.

At rest, glycolysis is ___________. During exercise, glycolysis is ___________.

inhibited. stimulated (to replenish ATP).

The amount of GLUT4 on the plasma membrane of muscle or fat cells is regulated by ___________ levels.

insulin. **Muscle and fat have increased glucose uptake only if insulin is present and binds to its receptor in these cells.

Pyruvate can be formed from muscle-derived lactate in the liver by _______________. The carbon skeletons of some amino acids can also be converted into gluconeogenic intermediates.

lactate dehydrogenase.

The major precursors for gluconeogenesis are _______________.

lactate, amino acids, and glycerol.

Low pH is not a regulator of PFK in liver, because only muscle produces and releases large amounts of __________ into the blood stream, but not liver.

lactate.

Cancer cells grow faster than blood vessels and are unable to obtain oxygen efficiently (hypoxia), thus _____________ is the primary source of ATP.

lactic acid fermentation.

High levels of ATP inhibit PFK in both muscle and liver. High levels of citrate inhibit PFK only in _________, since citrate signals that precursors are abundant.

liver.

Which can detect blood glucose levels, liver or muscle?

liver.

Major site of gluconeogenesis is the ____________, although some gluconeogenesis can occur in the kidney.

liver. **occurs in kidney during prolonged starvation.

Activity of phosphofructokinase increases when the ATP/AMP ratio is _____________.

lowered. **Glycolysis is stimulated as the energy charge falls.

Oxaloacetate is transported to the cytosol as ___________. Oxaloacetate is simultaneously decarboxylated and phosphorylated to _______________ in the cytosol by PEP carboxykinase.

malate. phosphoenolpyruvate (PEP).

Pyruvate carboxylase is a _____________ enzyme.

mitochondrial.

Main function of liver is to maintain ____________ during fasting.

normal blood-glucose levels.

First step in gluconeogenesis is the carboxylation of pyruvate to _____________, which occurs in the mitochondria

oxaloacetate.

Is glucose transport active or passive?

passive facilitated diffusion along a concentration gradient, does not need ATP.

The first irreversible reaction unique to the glycolytic pathway, the committed step is catalyzed by ______________.

phosphofructokinase.

F-2,6-BP activates ____________ by increasing its affinity for its substrate ____________ and by diminishing the inhibition by ATP only in liver.

phosphofructokinase. fructose 6-phosphate.

Inhibition of PFK leads to accumulation of fructose-6-P and to accumulation of glucose-6-P, because ______________ reaction is reversible. Accumulation of glucose 6-P leads to inhibition of hexokinase.

phosphoglucose isomerase.

The pyruvate kinase reaction of glycolysis is bypassed by ___________ and ______________ in gluconeogenesis.

pyruvate carboxylase and PEP-carboxykinase.

At low blood glucose levels, the glucagon-triggered cyclic AMP cascade leads to phosphorylation and inactivation of _____________.

pyruvate kinase.

Both isoforms of ____________ are activated by fructose 1,6-bisphosphate and inhibited by ATP and alanine.

pyruvate kinase.

Alanine is made from ____________.

pyruvate.

The gluconeogenic pathway converts _____________ into glucose.

pyruvate.

The L form of pyruvate kinase is regulated by _______________.

reversible phosphorylation.

The only difference in pyruvate kinase regulation in muscle versus liver is that the liver isoform of pyruvate kinase is also regulated by ______________.

reversible phosphorylation.

What are the 3 functions of glycolysis?

to generate ATP and provide carbon skeletons for biosynthetic pathways